Task 4 Flashcards
What does an MEG meassure ?
magnetic field produced by electrical currents (dipoles
What is the meassurment system of an MEG ?
femto tesla 10-15T
What are the two assumptions of MEG ?
- Can only measure electrical activity (current) that is parallel to the surface (tnagetial / Sulci neurons)
- Electric fields produced by the brain are really weak
- > room must be shielded with aluminum / mu metal
Whare are the sensors lcoated of an MEG ?
Helmet
-> Each sensor consit of 3 transformers and pick up coils
What is a Gradient coil ?
-> meassures in x / y direction
What is a Magnometer coil ?
-> meassures in z direction
How and where do the signals flow detected by the sensors ?
-> Flow via superconducting flux transformer
(increases SNR)
-> Where: Travel to SQUIDS those record magnetic field changes
Why is the MEG so large ?
Because the squids must be keept cold at 4 degrees
What is meant by the inverse problem ?
-> Calculating via a computer program the Source of The Magnetic Field based on a set of assumptions/observation
What are the assumptions of an inverse problem ?
- > Assume that brain is approx. spherical and that active areas can be represented by single/multiple current dipoles
- > Minimum norm estimates: Assumes to know the shape of the head, dipole orientation and distortion
What are Event related fields ?
- > Simmilar to ERP
- The average of MEG traces
Summarize EEG
- > Direct meassure of activation
- > is not limited to any neurons
- > high temporal resolution
- > limited spatial resolution
- > Cheap
SUmmarize MEG
- > Direct meassure of activation
- > is limited to tangential neurons
- > high temporal resolution
- > limited spatial resolution
- > Expensive
Summarize fMRI
-> Indirect measure of activation Represents changes in blood-oxygen levels (image is 4-6s behind) -> Bad temproal resolution -> High spatial resolution -> Expensive
Why would we still use EEG over MEG ?
- > Less practical complaints /portable
- > Sufficient spatial resolution
- > Cheap
Why is MEG better then EEG ?
-> beacuse it has a higher spatial resolution beacsue the skull is more transparent for magentic fields !
What are oscilaitions ?
-> The Brain waves which represent communication between spatially distributed bran networks
What is Local-scale synchronisation
-> synchronisation – among neighbouring neurons
What is Large-scale synchronisation
-> synchronisation – btw neuronal assemblies for distant brain regions
How can you intepreted oscilations ? (high frequency)
- > high frequency = originate from small neruons populations
- > high frequency = low amplitude
- > High degree of desyncronization
- > high frequency = brain has something to do (state effect)
How can you intepreted oscilations ? (Low frequency)
- > low frequency = originate from large neruons populations
- > Low frequency = High amplitude
- > Low degree of desyncronization
- > Low frequency = brain has nothing to do (state effect)
Describe the Delta band:
1- 4 HZ
- > Low frequency, high amplitude
- > with Sleep + Highly present in the first two years of our life
- > associated to proximity of brain tumors
Describe the Theta band:
4 -8 HZ
-> Low frequency, high amplitude
-> Mostly sleep but:
o First: widespread scalp distribution -> drowsiness & impaired information processing
o Second: frontal-midline –distribution -> focussed attention, mental effort & effective stimulus processing
Describe the Alpha band:
8-13 HZ
- > Relaxed wakefulness and cognitive inactivity
- > Lower alpha (8-10Hz) desynchronization -> stimulus unspecific expactancy
- > Upper alpha (10-12Hz) desynchronization -> processing of sensory-semantic information, stimulus-specific expectancy
